1. Field of the Invention
The present invention relates to a projection display system, and more particularly, to a projection display system including a diffractive linear display device. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for improving light use efficiency and enhancing brightness of a screen.
2. Discussion of the Related Art
Generally, a projection display system is a display system implementing a wide screen to display a wide image that is generated by projecting and enlarging a small image. An example of a conventional projection display system is an LCD (liquid crystal display) projection system that uses a lamp and an LCD. Such conventional LCD projection display systems create several problems.
For example, the switching response speed of the LCD is relatively low, which can create image artifacts when displaying a fast moving picture. In addition, when the liquid crystal display device operates to render a dark pixel in response to an off value of the electrical signal, the liquid crystal layer of the LCD display device generally is unable to completely block the light of the pixel that should not be illuminated. This dark state light leakage typically reduces the contrast of LCD projection display systems.
Conventional LCD projection display devices typically include optical systems with color separation and combination systems that increase the complexity and overall size of the projection display devices.
The high-temperature high-voltage lamp used as the light source can create safety concerns. In addition, since the expected life span of the lamp is about 5,000 hours, conventional LCD projection systems are not well suited for use in televisions or other products that should have a long life span. Finally, because the lamp has a wide spectrum of light, image color purity is reduced.
To solve these problems, a DMD (digital micro-mirror device), a GLV (grating light valve), or the like can be used in the projection display system instead of LCD. Also, the lamp can be replaced by an LED (light emitting diode), laser, or the like.
Specifically, the GLV, which is a diffractive linear display device, has a configuration of 100 ˜10,000 micro-ribbons formed by a semiconductor process. The GLV has a switching response speed much greater than that of the LCD, a contrast higher than that of the LCD and light efficiency better than that of the LCD. Hence, the GLV is more advantageous in implementing brighter images more naturally than the LCD.
FIGS. 1A to 1C are diagrams of a GLV (grating light valve) of a conventional MEMS (micro electromechanical systems) reflective display device. Referring to FIG. 1A, a set of six ribbons 110 and 120 forms one pixel. The ribbons 110 and 120 are alternately arranged. In this case, operational ribbons 120 are moved by an electrode 130, whereas fixed ribbons 110 are not moved by the electrode 130.
The grating light valve includes a linear array of pixels, or a pixel line. Each pixel in the linear array is formed by a set of six ribbons 110 and 120, such as those shown in FIG. 1A. The number of pixels in the linear array of pixels of the grating light valve is generally equal to the number of pixels in one of the two dimensions of the projection display device that uses the grating light valve.
In the GLV, if a voltage is not applied to the electrode 130, as shown in FIG. 1B, a mirror face is formed due to the surfaces of the ribbons being in the same plane. Hence, an incident light is reflected and returns without diffraction as reflected light that is parallel to and in the same path as the incident light.
In the GLV, if a voltage is applied to the electrode 130, as shown in FIG. 1C, the fixed ribbons 110 do not move, but the operational ribbons 120 are bent downward by an electrostatic force. Thus, this configuration of ribbons forms a grating with a periodic step shape in height, which causes diffraction of incident light.
In this case, the luminous intensity of the diffracted light can be modulated by adjusting the height of the displaced operational ribbons 120 or by selecting the time ratio of the up states to the down states of the operational ribbons 120 over a period of time. Hence, the GLV acts as a light modulator that adjusts the luminous intensity of the diffracted light.
FIG. 2 is a diagram of a general projection display system employing the GLV shown in FIG. 1. Referring to FIG. 2, a light emitted from a light source 200 is irradiated as a linear light to a GLV 100 by a linear optical illumination system 300. The light emitted from the light source 200 is a linear light in the sense that it is incident on the GLV 100 along a one-dimensional line that corresponds to the linear shape of the GLV 100.
The GLV 100 selectively diffracts the incident linear light to create a linear diffracted light. The GLV 100 operates in response to an electrical signal that encodes the image data that is to be displayed. The image is created in response to the electrical signal by adjusting the luminous intensity of the diffracted light of each pixel.
The light diffracted by the GLV 100 is passed through a lens 400 and is scanned by a scanner 500 to effectively convert the one-dimensional array of pixels to a two-dimensional array of pixels of a screen 700. A projection lens 600 enlarges the image to and projects the scanned light to the screen 700.
The diffracted light generated by the gratings of the GLV 100 includes a primary diffracted light mode, a secondary diffracted light mode, and higher-order diffracted light modes. Conventional grating light valves 100 use only the primary diffracted light mode, while the secondary diffracted light modes and the other higher-order diffracted light modes are lost. Hence, light use efficiency of conventional GLV projection display systems is relatively low.
Moreover, in the conventional GLV projection display systems, the secondary diffracted light modes and the higher-numbered diffracted light modes are scattered and affect the light path of the primary diffracted light mode, thereby degrading the contrast of the image.